Sharp MZ-80B Owner's Manual page 60

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4.3.2 Signal system for the 8253 block
The 8253 (programmable interval timer) works as built-in clock with its counters #0, # 1 and #2.
These counters are used as mode 2: rate generators, and are all 16-bit binary counters.
Counter #0 counts input pulses of 31.25 kHz, and delivers a pulse to OUT 0 every one second;
counter# 1 counts its output pulses, and delivers a pulse to OUT 1 every 12 hours; counter #2 counts
its output pulses, and repeats 0 and 1, thus working as AM/PM flag. See FIGURE 4.9.
8253
OUT2
Counter
GATE2
#2
CLK2
12 hours
OUT 1
8
D
Counter
GATE 1
17
#1
Do
CLK 1
1 second
auTo
Counter
GATEO
#0
CLKO
~+------- 31.25 kHz
FIGURE 4.9 Signal system for the 8253 block